On Mon, 30 May 2022 15:41:54 +0100, Mark Brown <broonie@xxxxxxxxxx> wrote: > > [1 <text/plain; us-ascii (quoted-printable)>] > On Sat, May 28, 2022 at 12:38:11PM +0100, Marc Zyngier wrote: > > On each vcpu load, we set the KVM_ARM64_HOST_SVE_ENABLED > > flag if SVE is enabled for EL0 on the host. This is used to restore > > the correct state on vpcu put. > > > > However, it appears that nothing ever clears this flag. Once > > set, it will stick until the vcpu is destroyed, which has the > > potential to spuriously enable SVE for userspace. > > Oh dear. > > Reviewed-by: Mark Brown <broonie@xxxxxxxxxx> > > > We probably never saw the issue because no VMM uses SVE, but > > that's still pretty bad. Unconditionally clearing the flag > > on vcpu load addresses the issue. > > Unless I'm missing something since we currently always disable > SVE on syscall even if the VMM were using SVE for some reason > (SVE memcpy()?) we should already have disabled SVE for EL0 in > sve_user_discard() during kernel entry so EL0 access to SVE > should be disabled in the system register by the time we get > here. Indeed. And this begs the question: what is this code actually doing? Is there any way we can end-up running a guest with any valid host SVE state? I remember being >this< close to removing that code some time ago, and only stopped because I vaguely remembered Dave Martin convincing me at some point that it was necessary. I'm unable to piece the argument together again though. M. -- Without deviation from the norm, progress is not possible.
